Low loss acoustic wave sensors and tags and high efficiency antennas and methods for remote activation thereof

ABSTRACT

Enhanced surface acoustic wave (SAW) sensors and SAW sensor-tag wireless interface devices, including low loss devices, devices that enable enhanced use of time diversity for device identification, and devices suitable for use in band-limited environments (such as ISM band) and for use in ultra-wideband applications are disclosed. Antennas for use with both SAW sensors and/or tags, and wireless transceiver systems also are disclosed, including antennas suitable for operation in conductive media and in highly metallic environments, said antennas being used to activate and read said SAW sensors and/or tags. SAW sensors and sensor-tags and related methods for measuring scaled voltage and current in electrical conductors via measurements of the electric and magnetic fields thereof are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/189,936, filed Jul. 8, 2015, herein incorporated by reference in itsentirety.

BACKGROUND OF THE INVENTION Field of the Invention

The field of the present invention relates to surface-launched acousticwave devices, including surface acoustic wave devices or SAW devices.The field of the present invention also relates to antennas for use withSAW devices, and the application of SAW devices and related methods forperforming measurements on electric and magnetic fields.

Description of Related Art

Acoustic Wave Sensors: Sensors based on surface-launched acoustic wave(known as surface acoustic wave or SAW) devices have been developedsince the 1980's for application to physical measurements (temperature,pressure, torque, strain, etc.) and to a wide range of chemical andbiological detection problems. These widely varying devices have beendescribed in detail in the open literature, including the following:U.S. Pat. No. 7,268,662, entitled Passive SAW-based hydrogen sensor andsystem, U.S. Pat. No. 7,431,989, entitled SAW temperature sensor andsystem, U.S. Pat. No. 7,500,379, entitled Acoustic wave array chemicaland biological sensor U.S. Pat. No. 7,791,249, entitled Frequency codedsensors incorporating tapers, U.S. Pat. No. 8,094,008, entitled Codedacoustic wave sensors using time diversity. U.S. Pat. No. 8,441,168,entitled SAW Sensor tags with enhanced performance, U.S. Pat. No.9,121,754, entitled Surface Acoustic Wave Deposition Monitor forUltra-Thin Films, U.S. Utility application Ser. No. 13/679,607(US20130130362A1), entitled Power Spectral Density Chemical andBiological Sensor, and U.S. Utility application Ser. No. 13/694,889(US20130181573A1), entitled Individually Identifiable Surface AcousticWave Sensors, Tags, and Systems.

Acoustic Wave Sensor Interrogation Systems: Acoustic wave sensor deviceshave been operated within a wide range of wired and wirelessinterrogation system architectures, which have generally been designedspecifically to operate with the selected sensor(s). The systemarchitecture is usually selected based on specific devicecharacteristics and application requirements, and generally involvesabsolute or differential measurements of sensor frequency, phase, delay,amplitude, or power spectral density, and changes in these quantitieswith exposure to changes in target parameters, to provide the outputsensor measurement. Conventional wireless interrogation systemarchitectures include pulsed radar-like delay measurement systems,Fourier transform based measurement systems, delay line andresonator-based oscillator systems, and time-integrating correlatorbased interrogation systems. Radio architectures include conventionalhomodyne and heterodyne mix-down systems, and direct (to baseband or tonear-baseband) conversion systems. Software defined radioimplementations of selected interrogation systems can be advantageous,in terms of flexibility and performance.

SAW multistrip couplers: Multistrip couplers (MSCs) are broadband, lowloss directional couplers that operate on freely propagating surfaceacoustic waves in substrates with high electromechanical couplingcoefficients. These components have been understood since the early1970's, as described by Marshall, Newton, and Paige in “Theory andDesign of the Surface Acoustic Wave Multistrip Coupler,” IEETransactions on Sonics and Ultrasonics, Vol. SU-20, No. 2, April 1973,pp. 124-133 and in “Surface Acoustic Wave Multistrip Components andTheir Applications,” IEE Transactions on Sonics and Ultrasonics, Vol.SU-20, No. 2, April 1973, pp. 134-143. MSCs have been widely used in SAWfilters and delay lines. However, advantages possible using MSCs in SAWsensors and sensor-tags have not been recognized in work found in theopen literature. MSC elements can be used to redirect the acoustic beam,to transfer acoustic energy from one track to another, to distribute theacoustic signal between a number of tracks, to enhance the operation oftapped delay lines, to reflect the acoustic signal (including creatinglow loss nearly ideal acoustic ‘mirrors’ that reflect broadband acousticsignals at a single point), for suppression of triple transit,suppression of bulk waves and spurious signals (as discussed by Danickiin “Bulk Wave Transmission by a Multistrip Coupler (MSC),” Archives ofAcoustics, 1998, pp. 125-137), and frequency modulation (among otherfunctions), to produce high rejection and/or low loss SAW devices, etherdesirable SAW device responses, and to perform numerous other usefulfunctions.

A few useful MSC configurations found in the open literature areillustrated here for reference. First, FIG. 1 shows a SAW device 100with a straight MSC 102 that is placed between an apodized transducer104 in a first (upper) acoustic track (apodization referring to thespatially varying overlap lengths between electrodes of oppositepolarities) and an unapodized transducer 106 in a second (lower)acoustic track. Voltages produced by the SAW in the upper track areintercepted by the MSC and are transferred to and spread uniformlyacross the lower track, generating a corresponding surface wave in thelower track. If the number of strips in the MSC is selected properly,the SAW launched to the right by the apodized transducer will be coupledover to the lower acoustic track with 100% efficiency. Bulk wavespropagating in the upper acoustic track will continue to propagate in astraight line, unaffected by the MSC, and any interference they may havecaused will be eliminated. Also, the MSC effectively spreads theacoustic energy from the apodized transducer over a uniform, fullaperture when launching the wave towards the second transducer. Thus, itis well known in the art that a MSC can be used between two apodizedtransducers that would not normally be cascaded in-line in a singleacoustic track. By adjusting the number of strips, the amount of energythat is transferred from the upper to the lower track can be adjusted.Different numbers of strips are required to effect a given % energytransfer between tracks on different substrates as well, based on theelectromechanical coupling coefficients of the materials involved. Thephase shift experienced by the SAW being transferred is also dependenton the number of strips and substrate material, in a manner that makesthe phase shift between tracks related to the percent of energytransferred. For 100% transfer, the SAW generated in the lower trackwill have a 180° phase shift relative to the SAW in the upper track,while for 50% energy transfer, the phase shift between tracks is 90°.

There can be variations of the straight MSC, including phase offsetsintroduced by jogging (or offsetting) the lines in different acoustictracks, transferring from one track into more than one track (of thesame or different acoustic apertures), and others. FIG. 2 shows amodified version of the straight MSC wherein the center region of theMSC is splayed out. This region corresponds to the portion of thesubstrate between the acoustic tracks. Angling of the MSC traces in thisregion prevents acoustic waves from being launched in a directioncollinear with the propagating acoustic waves in the upper and loweracoustic tracks in this unused region. Any launched waves would insteadbe launched off-angle.

A second known MSC structure that is useful is the U-shaped MSC. Thisstructure can be thought of as what would result if a straight MSC wasstretched and bent around so that two ends aligned, as shown in FIG. 3.The original straight MSC is a track changing MSC of known efficiency,so the SAW that would have been launched to the right in the lower trackis now launched to the left in the U-shaped MSC. The spacing in thecenter of the two arms of the “U” is chosen to ensure that 100% of thesignal is reflected to the left of the U shaped MSC. This structureproduces a broadband reflector that effectively reflects 100% of a SAWincident from the left as if there was an ideal acoustic mirror in thespace between the two arms of the U.

An alternate ring-shaped MSC reflector structure has certain benefits,discussed by Brown in “Low-Loss Device Using Multistrip Coupler RingConfiguration with Ripple Cancellation,” IEEE Ultrasonics Symposium,1986, pp. 71-76. Considering the bottom traces of the U-shaped MSC shownabove, the curved regions of the traces correspond to portions of theMSC that do not contribute to the production of SAWS the desiredacoustic track. Thus, the shorter these regions the better, butlithographic requirements necessitate a minimum separation space betweenadjacent traces, increasing size. In addition, since the spacing of theelectrodes will be on the order of 40% of an acoustic wavelength for theSAW of interest, when a SAW passes under the MSC and the first electrodeis near maximum SAW voltage, the adjacent electrode will be near aminimum voltage. Adjacent electrodes at alternating polarities willintroduce considerable inter-trace capacitance, which will alter theperformance of the MSC. An alternative ring-shaped MSC reflector isshown in FIG. 4, where the curved (or in this case segmented line) tracesections that connect two traces on either side of the U are alternatedso that every other curved section is on the opposite end of the MSC.This Changes the “U” shape to a ring, and ensures that most of thecurved segments at the top are one polarity while most at the bottom arethe other polarity, substantially reducing the inter-trace capacitanceintroduced by these sections of the traces. “Most” is used in thisdescription, since the non-synchronous nature of the trace spacingsrelative to the SAW wavelength means that the polarity within each end(top and bottom) will slowly vary within that region. But theimprovement in performance is significant.

SAW sensors incorporating dispersive elements: Dispersive elements suchas chirp transducers have long been used in SAW signal processingdevices such as radar expander/compressor pairs. These elements havealso been used in SAW sensors, as discussed in U.S. Pat. No. 8,441,168,entitled SAW Sensor tags with enhanced performance. Chirp elementsenable realization of processing gain, which can be effected on the SAWdie, or can be distributed to different portions of the system. Pulsecompression occurs when an upchirp signal is convolved with a downchirp(or vice versa). Alternate frequency coded pulses can also be used toproduce pulse compression. Chirp pulse compression takes a signal thatcontains frequency components from F_(low) to F_(high) (i.e., with abandwidth B=F_(high)−F_(low)) within a time length T, and by convolvingit with a device with the opposite chirp slope (i.e., if the inputsignal is an upchirp the compression filter is a downchirp and viceversa), transforms the signal in to a narrow pulse of duration t≈1/Bwith voltage amplitude increased by sqrt(BT). In SAW devices, it isquite easy to realize BT products of 50, 100, 200, or even more, whichcan produce significant processing gain.

SAW Sensor-tags: SAW devices have been used as wireless interfacedevices to external voltage producing or impedance varying sensors, asdiscussed in U.S. Pat. No. 8,441,168, entitled SAW Sensor tags withenhanced performance. SAW sensor-tag interface devices enable thewireless reading of (batteryless or passive) sensors that normally areoperated in powered, wired systems. SAW wireless interface devices,which can be individually identifiable, or RFID-enabled, are referred toas SAW sensor-tags or SAW sensor-tag wireless interface devices.Brocato, in “Passive Wireless Sensor Tags,” Sandia Report SAND2006-1288,Sandia National Laboratories, Albuquerque, N. Mex. 87185, March 2006,demonstrated that a SAW differential delay line could be used, with asensor that changes impedance with measured quantity attachedelectrically in parallel with a reflector in one of the paths, tomeasure variations in the attached sensor. Other researchers have alsodemonstrated similar devices, including those described by Reindl et al.in “Theory and Application of Passive SAW Radio Transponders asSensors,” IEEE Transactions on Ultrasonics, Ferroelectrics, andFrequency Control, Vol. 45, No 5, September 1998. pp. 1281-1292, Schollet al in “Wireless Passive SAW Sensor Systems for Industrial andDomestic Applications,” Proceedings of the 1998 IEEE InternationalFrequency Control Symposium, p. 595-601, and Schimeita et al in “AWireless Pressure-Measurement System Using a SAW Hybrid Sensor,” IEEETransactions on Microwave Theory and Technique, Vol. 48, No. 12,December 2000, pp. 2730-2733.

More recently, a range of RFID enabled, dispersive, half-dispersive, andnon-dispersive SAW wireless interface devices have been demonstratedthat can be used to read a wide range of external sensors and devices,including switches, RTDs, thermistors, strain gauges, and acousticemission sensors. Methods have been shown for adapting SAW sensors tooperate with external sensors with impedances (Z) having real partsRe(Z) varying from low (˜20Ω) resistance to moderately high (over 5 kΩ)resistance, the latter being devices that normally would not interfacewell with SAW devices. This work is described in the final report forNASA SBIR Phase I contract NNX09CE49P (Jul. 22, 2009). For voltagegenerating external sensors (such as AE sensors, thermocouples, etc.),the external sensor voltage is applied as the gate voltage on azero-bias (normally ON) field effect transistor (FET).

SUMMARY OF THE INVENTION

The present invention provides for SAW sensor and sensor-tag deviceembodiments that exhibit lower loss and/or enhanced time diversityperformance relative to conventional systems. Antennas suitable forefficient operation its conductive media are provided. Sensors that canmeasure electric current and scaled voltage (through measurement of themagnetic field and electric field, respectively) are also disclosed.

Low-loss differential delay line SAW sensors using MSC elements:Conventional SAW differential delay line sensors often utilize SAWtransducer and reflector elements and die layouts that result in lossmechanisms that include bidirectional loss from the transducers,inefficiency in reflector operation, and power division loss due tomultiple acoustic tracks. All of these factors tend to reduce the signalstrength of each reflected sensor response relative to the input signalstrength. This negatively impacts the signal to noise ratio (S/N) andreduces the wireless range over which such devices can be read using agiven radio and set of antennas. Higher loss can also negatively impactthe accuracy of measurements produced by the sensor and sensor-tagdevices, for a given radio interrogation system. The present inventionteaches the use of MSC elements in novel device embodiments that producedevices with lower loss relative to conventional devices.

Low-loss differential delay line SAW sensors with enhanced timediversity: Time diversity is a property of sets of SAW sensors and tagsthat provides for each device to respond within a separate, distincttime range when the set of devices is activated by a common radiofrequency (RF) interrogation pulse. The present invention teaches SAWsensor and sensor-tag device embodiments that incorporate dispersiveand/or MSC elements to produce responses that are shorter in time andlower loss than conventional devices capable of producing an equivalentnumber of separately detectable responses. These properties enableconstruction of sets of passive wireless sensors have more timediversity within a given total time length, enabling production ofsensor systems with more sensor or sensor-tag devices simultaneouslyoperable without inter-device interference. The lower loss of devicesaccording to the present invention either increases the wireless rangewith which the devices can be read, or improves the accuracy ofmeasurements produced by each device, or a combination of both.

Inductive coupling and resonant magnetic coupling for remote reading ofSAW sensors: Most conventional wireless SAW sensors have been activatedvia conventional radiative antennas, which interact via propagating,time varying electromagnetic signals. Such antennas do not functionproperly when mounted on metal surfaces, or when immersed in conductivemedia. Some SAW devices for use in liquids have been demonstrated thatuse inductive coupling to activate the SAW device wirelessly at shortrange (up to a cm or two). The present invention teaches the use ofresonant magnetic antennas for remote activation and reading of SAWsensors and sensor-tags, over longer-range, in highly metallicenvironments, and/or when immersed in conductive media.

Wireless measurement of current and voltage: The present inventionteaches devices, apparatuses, systems, and methods involving the use offield probes with FETs and SAW sensor-tag wireless interface devices tomeasure voltage and current in current carrying conductors (CCCs) viameasurement of the electric fields and magnetic fields around said CCCs.The present invention also teaches devices, apparatuses, systems, andmethods for determining the relative phase (leading or lagging) of thecurrent and voltage in AC power systems, thereby providing theinformation necessary to determine power factor. Such information isuseful, for example, in monitoring the condition of the powerdistribution grid.

Aspects of the present disclosure include an apparatus for wirelesslymeasuring the strength of electric and/or magnetic fields. The apparatusincluding one or more acoustic wave sensor or sensor-tag devices, atleast one transistor, at least one field probe operable to interact withthe held being measured and produce a voltage, and one or more antennas.

Further aspects of the present disclosure include a surface acousticwave Sensor tag device. The devices includes a piezoelectric substrate,at least one first transducer arranged on at least a portion of saidpiezoelectric substrate, and at least one second surface acoustic waveelement formed on said piezoelectric substrate and spaced from saidtransducer along the direction of acoustic wave propagation, whereinsaid at least one second surface acoustic wave element includes at leastone multistrip coupler (MSC).

Additional aspects of the present disclosure include an antenna forremote activation and read of SAW sensors. The antenna includes at leastone pair of magnetic coupling elements operable to create a magneticresonance therebetween, wherein said at least one pair of magneticcoupling elements can transfer energy and data to and from SAW devicesimmersed in conductive materials.

Still other aspects, features, and advantages of the present inventionare apparent from the following detailed description, simply byillustrating exemplary embodiments and implementations, including thebest mode contemplated for carrying out the present invention. Thepresent invention also is capable of other and different embodiments,and its several details can be modified in various respects, all withoutdeparting from the spirit and scope of the present invention.Accordingly, the drawings and descriptions are to be regarded asillustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given below and from the accompanying figures and drawingsof various embodiments of the invention, which, however, should not betaken to limit the invention to the specific embodiments, but are forexplanation and understanding only.

FIG. 1 shows a schematic representation of a conventional SAW deviceincorporating a straight track-changing MSC.

FIG. 2 shows a schematic representation of a modified straight MSC thathas a center region (between the acoustic tracks) where the traces havebeen angled to prevent acoustic waves from being launched collinear tothe waves hi the upper and lower acoustic tracks.

FIG. 3 shows a straight MSC being bent to produce a conventionalU-shaped MSC.

FIG. 4 shows a conventional ring shaped MSC.

FIG. 5 shows a partial view of a photomask layout for a set ofdifferential delay line SAW sensors that utilize conventional SAWtransducer and reflector elements.

FIG. 6 shows a schematic representation of a low-loss SAW differentialdelay line sensor incorporating ring type MSCs according to aspects ofthe present disclosure.

FIG. 7 shows a schematic representation of another embodiment of alow-loss SAW differential delay line sensor incorporating ring type MSCsand more than one acoustic track according to aspects of the presentdisclosure.

FIG. 8 shows a schematic representation of an embodiment of a twoacoustic track low-loss SAW differential delay line combined sensor andsensor-tag device incorporating both ring type MSCs and externalimpedance varying device(s) according to aspects of the presentdisclosure.

FIG. 9 shows a schematic representation of a low-loss SAW differentialdelay line sensor embodiment incorporating track changing MSCs and ringtype MSCs according to aspects of the present disclosure.

FIG. 10 shows a schematic representation of a low-loss SAW differentialdelay line SAW sensor embodiment according to aspects of the presentdisclosure.

FIG. 11 shows another schematic representation of a low loss SAWdifferential delay line sensor embodiment according to aspects of thepresent disclosure.

FIG. 12A shows a schematic representation of an additional SAW deviceaccording to aspects of the present disclosure.

FIG. 12B shows a schematic representation of an additional SAW deviceaccording to aspects of the present disclosure.

FIG. 12C shows a schematic representation of an additional SAW deviceaccording to aspects of the present disclosure.

FIG. 13 shows a schematic representation of a low-loss SAW device withdispersive elements and track changing MSC reflectors that uses pulsecompression according to aspects of the present disclosure.

FIG. 14 shows a schematic representation of a low-loss differentialdelay line SAW sensor with dispersive elements and track changingreflective MSC elements that uses pulse compression according to aspectsof the present disclosure.

FIG. 15 shows a schematic representation of a device with track changingMSCs and ring-shaped MSC reflector elements and dispersive elements thatuses pulse compression according to aspects of the present disclosure.

FIG. 16 shows a schematic representation of a compact, single acoustictrack low loss differential delay line sensor using pulse compressionaccording to aspects of the present disclosure.

FIG. 17A shows a cut-away view of the interior of a voltage and currentmonitoring system according to aspects of the present disclosure.

FIG. 17B shows the mechanical housing enclosing the conductor for thesystem shown in FIG. 17A according to aspects of the present disclosure.

FIG. 18 shows the system of FIGS. 17A and 17B in operation on a threephase power line according to aspects of the present disclosure.

FIG. 19 shows a schematic representation of a wireless, batteryless SAWcurrent sensor-tag assembly according to aspects of the presentdisclosure.

FIG. 20 shows a schematic representation of a wireless, batteryless SAWvoltage sensor-tag assembly according to aspects of the presentdisclosure.

FIG. 21 shows a SAW wireless interface device configuration withmultiple FETs for use in monitoring current (magnitude and direction)and voltage (magnitude and polarity) according to aspects of the presentdisclosure.

FIG. 22 shows the circuit of FIG. 21 with added transient voltagesuppression (TVS) devices across the two leads of the field probeaccording to aspects of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention teach devices, apparatuses,systems, and methods for effectively increasing the signal strength(i.e., reducing the loss) and/or providing enhanced time diversityperformance of SAW sensor and sensor-tag devices relative toconventional devices. Antennas suitable for efficient operation inconductive media are disclosed. Sensors that can measure current andscaled voltage (through measurement of the magnetic field and electricfield, respectively) are also disclosed.

Low-loss differential delay line SAW sensors using MSC elements:Conventional SAW differential delay line sensors utilize bidirectionaltransducers with ordinary reflectors that are either short circuited oropen circuited transducers or arrays of electrodes. The partial masklayout shown in FIG. 5 illustrates and array of devices (or die) such asthe one labeled 200, each with two input transducers 202 (on the leftend of each die) that are electrically fed in parallel, and two ordinaryreflectors located 204 at different distances and hence at differentacoustic delays to the right. SC and OC in the labels on each dieindicate short circuit and open circuit reflectors, respectively. Thesedevices suffer from loss mechanisms that include bidirectional loss fromthe transducers and inefficiency in reflector operation. In addition,feeding two transducers in parallel causes a power division ‘loss’,reducing the signal strength in each acoustic path relative to the inputsignal strength. Unidirectional transducers can be used to reduce thebidirectional loss, as they send and receive acoustic waves in only onedirection.

Selected disclosed apparatuses, devices, systems, and methods usering-shaped MSC reflectors such as those described above to providereflectors that are nearly 100% efficient, significantly reducing thelosses due to inefficient reflectors. An added benefit of using thesereflectors is that the reflected SAW response is not broadened in timeby the reflector as would be the case with a normal reflector. Thereflected response appears as if it were reflected from a nearly idealmirror located at the center of the ring MSC reflector. This isbeneficial for SAW sensors utilizing time diversity as one factor indevice identification. Some device embodiments are provided below.

The first embodiment, shown schematically in FIG. 6, is a simpledifferential delay line using a bidirectional transducer and two ringMSG reflectors located on either side. The SAW device 210 includes atransducer 212 located between two ring MSC reflectors 214, which arelocated at different distances from the transducer. When impulsed, thisdevice will produce two responses at different delays, determined by theround trip spacing between the transducer and each reflector and theacoustic wave velocity. Clearly, this structure can be modified in anumber of ways to make useful sensors. The delays can be adjusted toproduce two closely spaced responses that produce a bandpass responsewith one or more notches in the passband. Alternately, films or othersurface treatments can be introduced onto portions of the propagationpath on one or more regions of the surface to produce sensors suitablefor detection of chemical, biological, or other analytes. Physical diemounting and packaging techniques and other methods can also be used toadapt such a device to use as a physical sensor for an assortment ofparameters. Construction of a device with multiple acoustic tracks, eachcontaining a differential delay line such as that shown above, canproduce sensors capable of making multiple measurements that are usefulfor many purposes. Addition of multiple transducers in the acoustictrack (or tracks) can provide a means a connecting external sensors tothe SAW die, providing a wireless interface device to said externalsensor or sensors, while also potentially providing one or more SAWsensor measurements. This type of structure can be useful formeasurement of multiple parameters, such as temperature (measured by theSAW die) and strain (measured by an external strain gauge and readthrough the SAW response or measured by the SAW die), voltage (measuredby the SAW die or measured using a FET external to the SAW and readthrough the SAW die) and/or acceleration (read through the SAW as awireless interface device). Various diversity techniques can beincorporated in these devices, including coding such as chirp/dispersivecoding, direct sequence spread spectrum (DSSS) coding, and other knowntechniques.

Another embodiment is shown in FIG. 7, where a two acoustic track device220 with two transducers 222 on the left (which can be unidirectionaltransducers) launch SAWs towards two ring MSC reflectors 224, which arelocated at different acoustic delays on the surface. Acoustic waveslaunched by the transducers propagate to the right on the die and arereflected back at time delays determined by the spacing of the SAWelements on the die surface, and the acoustic wave velocity. Thetransducers are shown sharing a common bus bar, but this is not requiredfor device construction and operation. This can be extended to mulitpletracks beyond two, and used with additional SAW elements such astransducers that can be connected to external sensors, or with ordinaryreflectors that are partially reflective, to allow multiple responsesfrom the region on one side of a transducer in one acoustic track. Withmultiple transducers in one acoustic track, these devices can be used inreflective mode, where the S11 response of the device is measuredthrough one port attached to an antenna, or in a reflective transmissionmode where the S21 response through two or more transducers attached tothe same antenna is measured. For clarity, if the two (or more) inputtransducers are formed with busbars that are independent, one of thesetransducers can be used as the interface to the external sensor.Alternatively, other transducers can be added to serve this purpose.

FIG. 8 shows just one example of a sensor-tag embodiment, where onetransducer has been attached to an external impedance varying load.Voltage producing external sensors and other devices can also be readusing a FET as an Interface device (see current and voltage sensingsection below). In FIG. 8, SAW device 230 includes three transducers232, one of which 234 is connected to an external impedance varying load236, and multiple (in this example four) U-shaped MSCs 238.

Another MSC element is the track changing MSC. A simple SAW differentialdelay line sensor embodiment 240 that incorporates a track changing MSCelement 242 is shown in FIG. 9. In the configuration shown, one wouldnormally construct the MSC 242 to be a 50% track changing coupler, sothat half the acoustic energy propagating to the right in the uppertrack from transducer 244 continues on to the far MSC ring reflector246, while the other 50% changes to the lower track, where it reflectsfrom the closer MSC ring reflector 248. This type of track changingstructure can be used on one side of a bidirectional transducer, or atwo-sided device can be constructed with track changing MSCs on one orboth sides. Other variations, such as track changing MSCs with unequalacoustic apertures, devices with more than two acoustic tracks, andtrack changing MSCs with delay offsets and/or subharmonic or harmonicspreading of the MSC traces are beneficial for certain purposes.

FIG. 10 shows a schematic representation of a low-loss SAW differentialdelay line SAW sensor that incorporates both track changing MSCs andring shape MSCs, in addition to transducers and reflectors of variouskinds, resulting in folded acoustic tracks that have longer acousticdelays for some of the reflective SAW responses than would otherwise bepossible for a SAW die of a given size using conventional devices.Specifically, FIG. 10 shows five individual SAW die 250. On each die asingle transducer 252 acts as an input/output (I/O) transducer,launching acoustic waves upward and downward. The upward propagatingwave is reflected the reflector 254 at the upper right of the device (inthis example, an apodized reflector with open circuited electrodes,although another die shows an unapodized reflector 256). This reflectedresponse returns to the launching transducer, providing a firstresponse. The acoustic wave launched downward propagates to the MSC 258below, which is a 50% track changing MSC. Half of this signal passesdownward through the right of the MSC 258, and reflects off thering-shaped MSC reflector 260 at the bottom right. This reflectedresponse is then effectively shunted to the left by the track changingMSC 258, as the portion that would pass through in an upward directionon the right is cancelled but an out of phase contribution from thesignal reflected off the ring-shaped MSC reflector 262 on the left. The50% of the initial signal that is shunted to the left track 266 islaunched downwards with a 90° phase shift relative to the wave in theright track 268. This propagates down, is reflected off the lower ringreflector, and returns to the track changing MSC. The relative phases ofthe waves result in no upward propagating wave in the right track, withall of the propagating energy going upward in the left track. It isreflected from the reflector at the top left 264 (again in this examplean apodized reflector), and returns downward. This wave goes throughanother pass of being track changed, reflected by the ring MSCreflectors 260 and 262 at the bottom, and is finally recombined in theright track, heading upwards towards the I/O transducer where it createsa second response. This device embodiment provides the ability tosignificantly increase the differential delay in a sensor device for agiven die size, as the second response occurs after the wave propagatesfrom the I/O transducer to the lower ring MSC reflectors, back to thetop of the die, back to the ring MSC reflectors, and back up to the I/Otransducer. Thus the overall delay of the second reflection correspondsto almost three times the distance between the bottom and topreflectors—a significant increase relative to simple differential delayline structures shown in FIGS. 6 through 9. As sensor sensitivity for adifferential delay line (excluding notch devices) depends, in part, onthe differential delay—with longer delays producing sensors with greatersensitivity, the ability to ‘fold’ the acoustic response on the diesurface (using this or other known MSC structures) can produce sensorswith improved response sensitivity. Finally, the fact that thisstructure requires only one transducer as an input/output transducermeans that there is no power division loss as would occur when two ormore transducers are electrically connected in parallel. This reducessensor loss further. With proper design, this embodiment (among othersdescribed in the present disclosure) can be designed to produce multipletransit responses from the near reflector 256 that occur at relativelyshort delays, providing three or more different delay responses that canbe used to eliminate phase ambiguity and enhance precision ofmeasurements extracted from the sensor or sensor-tag devices.

FIG. 11 shows another schematic representation of a low-loss SAWdifferential delay line sensor that incorporates multiple transducers,making it useful either as a SAW sensor or a SAW sensor-tag wirelessinterface device that can be used to read external devices connected toone or more of said transducers, or as a combined device where selectedmeasurements are made by the SAW device itself while other measurementsare made by external devices and wirelessly read through the SAW deviceas an interface. Specifically, this device 270 includes threetransducers 272, a track changing MSC 274, and a ring-shaped MSCreflector 276. This device 270 can be used as a sensor-tag, with one ormore of the transducers attached to external sensors and othercomponents (such as impedance transforming components and FETs) toproduce acoustic responses that depend on the external devicemeasurements.

FIGS. 12A-12C show schematic representations of three additional deviceembodiments according to aspects of the present invention. Referring toFIG. 12A, SAW differential delay line device 280 includes two or moretransducers 282 in one acoustic track, and two or more reflectors, shownas ring reflectors 284. This device 280 can be used in a transmissionmode, where the transducers 282 are connected electrically in inparallel to a common antenna, and the reflected response is thecombination of the S21 and S12 transmission responses. Alternatively,one transducer can be used as an I/O transducer, and the othertransducer can be used as a reflector or interface to an externalimpedance varying or voltage producing device. The spacings 286 (betweenthe two transducers), and 288 and 290 (between the transducers and thereflectors) can be varied to produce devices with a range of performanceproperties. The acoustic delay corresponding to spacing 286 determinesthe time at which the first transmission response occurs, and the timesat which multiple transits of this response occur. Acoustic delays dueto spacings 288 and 290 can be equal, slightly unequal, or very unequal.Equal spacings 288 and 290 would produce reflections that add in phaseat the output. Due to multiple transit effects, this device 280 wouldstill produce multiple responses, which can be useful. Slightlydifferent spacings 288 and 290 will produce two responses slightlyoffset in time, which can result in a combined frequency domain responsethat has a power spectral density (PSD) with one or more notches.

Referring to FIG. 12B, SAW device 292 consists of a single transducer294 within the center region of a ring-shaped MSC 296. Referring to FIG.12C, SAW device 300 includes two or more transducers 310 within aring-shaped MSC 320. Both devices 292 and 300 function as resonantdevices, one-port and two-port respectively. Added spacing between thetransducers 310 in device 300 can be used to add an initial acousticdelay to the device response. Such a resonant or tinging response withan added initial acoustic delay can also be implemented using twotransducers between two efficient reflectors (such as ring-shaped MSCs)on either side of two transducers, similar to FIG. 6 with twotransducers rather than one. Additional embodiments can include morethan two transducers.

This is just a small subset of possible device embodiments, as thisapproach can be extended to include multiple acoustic tracks, singlesided die with UDTs or double sided die (on one or more tracks), andmany other configurations that incorporate alternative MSC structures(fanned MSCs, J-shaped MSCs, interleaved U-shaped MSCs, and manyothers). Within the scope of the present disclosure, any of theseembodiments can also include additional transducers in various locationson the surface, which can be used as interfaces to external impedancevarying or voltage producing sensors, to produce wireless interfacedevices (or sensor-tags). Other embodiments within the scope of thepresent disclosure can include additional reflectors and MSC elements,multiple acoustic tracks, and transducers, or combinations thereof. Theresulting devices can be used as SAW sensors, where all measurements aremade directly by the SAW die. Alternatively, the resulting devices canserve as sensor-tags, as wireless interface devices to externalmeasurement devices that have varying impedance or produce voltages inresponse to changes in measured parameters. Or, the resulting devicescan be hybrid devices, where selected measurements are made directly bythe SAW die and other measurements are made by external devices and readwirelessly through the sensor-tag device to which the external devicesare connected.

Low-loss differential delay line SAW sensors with enhanced timediversity: Much recent work has focused on developing sets of passivewireless sensors that are non-interfering, so that calibratedmeasurements can be taken from sets of sensors that are operatingsimultaneously in the field of view of a transceiver. Time diversity,frequency diversity, code diversity, and/or chirp diversity have beenused in combinations that make devices both individually identifiableand non-interacting. Depending on the bandwidth that is available in agiven application environment, these diversity techniques can becombined in a manner appropriate to achieve sets of sensors that worktogether well. One way to enhance the time diversity capacity of asensor set is to shrink the time length of each SAW sensor responsepulse, since pulses that are confined within a narrow time range allowmore of said pulses to fit within a given time range. However, forconventional finite impulse response (FIR) SAW filters, the time lengthof a transducer is inversely related to its bandwidth—meaning that veryshort transducers are broadband—which reduces the capacity for frequencydiversity. Additionally, very broadband signals may not fit withinallowed FCC frequency ranges for radio frequency (RF) electronictransmissions (such as in the ISM band). Also, there is another tradeoffwhen using very short transducers—each additional electrode overlapcouples more energy into the SAW substrate. So using very shorttransducers can reduce the overall efficiency of electromechanicalcoupling, increasing device loss. Thus, physics limits the tradeoffsthat can be made when trying to realize diversity schemes forconventional SAW structures.

The devices disclosed herein, by comparison, take advantage of the timecompression and signal processing gain introduced by pulse compressionto produce SAW responses that are very narrow in time and simultaneouslyhave significantly increased signal levels—improving both time diversityand reducing loss. FIG. 13 shows one SAW differential delay line device320 according to aspects of the present disclosure. In this device 320,two transducers 322 with opposite chirp slopes are positioned in twoacoustic tracks. On each end are located reversing MSC elements 324 thatboth track change with 100% efficiency and also change the direction ofthe reflected wave. When the two transducers 322 are electricallyconnected in parallel, acoustic waves are simultaneously launched to theleft and right in both the upper and lower acoustic tracks. The signallaunched to the left in the upper acoustic track, which is a downchirp(the high frequency comes first and the low frequency later) is coupleddown to the bottom acoustic track, where it then is effectivelyreversed, propagating to the right. When received by the transducer inthe lower track (an upchirp—which has the low frequency first and thehigh frequency later), there is very little output signal until thechirp signal is fully in the transducer, at which point a strongconvolution response peak (compressed pulse) is generated.Coincidentally, the wave launched to the left in the lower track is anupchirp. This is reflected, reversed, into the upper track where it iscompressed by the downchirp in the upper transducer. The two compressedpulses from the two propagation paths on the left occur simultaneouslyand add. A similar process involving the acoustic waves launched to theright in the upper and lower acoustic tracks produces a secondcompressed pulse from the acoustic propagation path and reflector on theright.

Thus, with the transducers fed in parallel by an antenna, as isconventional for SAW sensors, any activating signal will generatesignals in the upper and lower tracks, which are counter-propagating,and recombine in transducers to produce a pair of output compressedpulses that have significantly stronger signal strength than the inputsignal, and also are very short in time. As previously discussed, SAWdevices with high BT products can transform fairly long duration signalsinto pulses that are much narrower in time. Pulse compression canproduce pulses of duration t≈1/B with voltage amplitude increased bysgrt(BT). By way of example, a BT of 100 can be implemented in a SAWtransducer that is 2 microseconds long with a bandwidth of 50 MHz, amongother combinations. This BT will generate a compressed pulse that is 20nsec wide, with an amplitude that is 10 times larger than the originalsignal—introducing 20 dB of processing gain while reducing the timeextent by a factor of 50.

Groups of sensors designed to operate as differential delay lines can bedesigned with response pulses interleaved in time, with individualsensor responses extracted through proper time domain windowing inaddition to windowing in frequency and convolving with coding iffrequency and code diversity are also used). Inclusion of frequencydiversity and/or code diversity can increase the total sensor set sizepossible or a given bandwidth and degree of time diversity.

The transducers 322 can be offset relative to one another (let to right)as shown, or vertically aligned. Also, the transducer pair can bepositioned centrally, so that reflections from both sides arrive back atthe transducers at the same time producing one large output pulse, orthe pair of transducers 322 can be offset to one side or the other. Ifthe transducers 322 are aligned vertically, and offset to one side ofcenter, this produces differential delay line that produces two strongoutput pulses at different times. A similar differential delay lineresponse occurs when the transducers are not aligned vertically (i.e.,they are offset left to right relative to one another) independent oftheir placement between the two reversing MSC elements, and/or when theyaligned vertically and are off-center relative to the two reversing MSCelements.

One example of a track changing, 100% reflective MSC is that shown byDanicki in “Theory of Surface Acoustic Wave Reversing MultistripCoupler,” Archives of Acoustics, January 1994, pp. 227-238. FIG. 14shows a schematic representation of a low-loss differential delay lineSAW sensor 330 according to aspects of the present disclosure thatincorporates this structure. The sensor 330 also includes chirptransducers 334 with chirp senses as shown, or equivalently the twochirps shown could both be mirrored left to right, or they could beswapped between upper and lower acoustic tracks, all with nominallyidentical results. For any of these configurations, the overalloperation of this embodiment is as described for the device 320 in FIG.13. In the reflective track changing reversing MSC elements 332, the MSCstrips are spaced so they are 120° apart, so that swapping two of everythree traces as shown reverses the direction of acoustic wavepropagation in the second track relative to the wave propagation in thefirst track. (It should be noted that close attention should be paid toMSC line spacing to ensure that the lines are not synchronous, whichwould produce reflections of the acoustic wave).

Another way to implement reversing track changing MSC elements accordingto the present invention would be to combine a track changing MSC withring-shaped MSC reflector elements in an appropriate configuration. FIG.15 shows one such device 340 embodiment that includes two transducers342 with opposite chirps, two track changing MSCs 344 that wouldnormally be 50% track changing MSCs, and four ring-shaped MSC reflectors346. As above, the center transducers can be fed in parallel and can beseparate or electrically connected on the die (as shown). Additionalcomponents could be added to enable this structure to be used as asensor-tag interface device.

A more compact, single acoustic track low loss differential delay linesensor 350 using pulse compression according to the present invention isshown FIG. 16. This embodiment uses a symmetrically chirped centertransducer 352, surrounded by two ring-shaped MSC reflectors 354. Thedispersive signal sent to the left (an upchirp followed by a downchirpis reflected, and generates a strong compressed pulse that is short intime duration when it is again time aligned with the input transducer.The same process happens to the right. Once again as a single transducerinput device, there is no power division loss. Processing gain enhancesoutput sensor signal levels considerably (easily by 20 db or more).

In the example device 350 shown in FIG. 16, a smooth upchirp is followedby a time-reversed downchirp. But any time-symmetric dispersivetransducer construction could be used to similar effect, including stepchirped, frequency coded chirped (wherein sections of the frequencychirp are ‘cut’ and ‘shuffled’ in time), nonlinear chirps, chirps ofvarying chirp slope, and others. Codes can also be introduced asmentioned previously, though for use in this structure any codes used inthe transducer 352 must also be time symmetric.

Other devices according to aspects of this invention include embodimentsthat contain multiple acoustic tracks with duplicate copies of thestructures above, with the same or different delays, and/or with otheradded SAW elements such as transducers, transducers attached to externalsensor elements, reflectors (conventional or ring MSC reflectors), MSCs,and films or surface treatments to produce waveguiding, to short out theelectric field at the surface, or to introduce sensitivity to a targetmeasurand (as in magnetostrictive films, biological moieties, andchemically sensitive and selective films). The device embodimentsdescribed can be implemented using any of a number of surface-launchedacoustic waves, including Rayleigh waves, flexural plate waves (FPWs),Love waves, surface transverse waves (STW), shear horizontal acousticplate modes (SH-APM), layer guided acoustic plate modes (LG-APM), layerguided shear horizontal acoustic plate modes (LG-SHAPM), leaky surfacewaves, and pseudo-SAW modes. The transducers and/or reflectors describedcan be tapered, slanted, stepped tapered, apodized, withdrawal weighted,EWC, UDT, SPUDT, dispersive, and/or waveguide structures. All of thesetechniques can also be used incorporating dispersive and harmonictechniques.

The broad nature of the embodiments described here are clear, and oneskilled in the art will understand that there is a wide variety ofdevice configurations that can be generated using combinations of one ormore of the techniques discussed. The embodiments of the inventionsdescribed herein and illustrated in the figures provide deviceembodiments capable of monitoring deposition of a wide range ofmaterials, including but not limited to ultrathin films andnanomaterials. While some preferred forms and embodiments of theinvention have been illustrated and described, it will be apparent tothose of ordinary skill in the art that various changes andmodifications can be made without deviating from the inventive conceptsset forth above.

Inductive coupling and resonant magnetic coupling for remote reading ofSAW sensors: To date, most wireless SAW sensors described in theliterature have been activated via conventional antennas. Dipole,monopole, patch, microstrip, slot, and numerous other antennaconfigurations have been used to activate SAW sensors via conventional‘radiative’ antennas that interact with propagating, time varyingelectromagnetic (EM) signals. Some SAW devices for use in liquids havebeen demonstrated that use inductive coupling to activate the SAW devicewirelessly at short range (up to a cm or two). A range of applicationsexist for which it would be advantageous to produce wireless SAW sensorsthat can operate in media that are slightly to moderately conductive(such as sea water or heavy brine), and also in highly metallicenvironments. Conventional radiative antennas do not function properlywhen mounted on metal surfaces. EM signals experience excessively highloss in conductive or partially conductive media, as the conductivitytends to short out the electric field portion of the wave. Thus,alternative methods of activating and reading the sensors were needed.Since magnetic fields are not ‘shorted out’ by the conductivity of themedia in which it exists, magnetic methods for transfer of power aremore useful in high conductivity environments.

Resonant magnetic coupling has been demonstrated for power transfer bycompanies such as Witricity, enabling highly efficient wireless powertransfer over several meters (in air). Magnetic communication of powerand data through metal has also been demonstrated using magneticsignals. For example, Hydro-Tech (Corey Jaskolski, President) has usedmagnetic signals to transfer power and sensor data through metalcylinders. Resonant magnetic coupling once again provides a moreefficient power transfer.

Certain aspects of the present invention disclose magnetic ‘antennas’such as coils (planar and helical), magneto-electric monopoles anddipoles, with appropriate matching components, to activate and read SAWdevices (sensors and sensor-tags, among others) in conductiveenvironments. Various electrically insulating materials can be used toisolate the antenna from the surrounding environment. Helical coils canbe circular, oval, rectangular, square, or other shape in cross section,and planar loops and coils can also be of many shapes, such as shapessimilar to helical inductors among others. Other aspects of the presentinvention combine magnetic power transfer with electrical loadmodulation of the magnetic power signal for transfer of data. Magnetic‘antennas’ can be mounted on or very close to metal surfaces, and can beused to activate SAW sensors and to receive sensor data. Such a methodcan operate in conductive media and on, around, and potentially throughcertain metal surfaces. Operation of SAW sensors in conductive fluidshas been successfully demonstrated using aspects of the presentinvention.

Wireless measurement of current and voltage: SAW devices were shown inthe early 1970's to be capable of measuring voltages directly, throughthe change in acoustic wave velocity produced by voltages appliedtransverse to the SAW die. However, the voltages required to producesignificant changes in frequency or delay were large (hundreds of voltsto kV) for realistic die thicknesses. More recent work has shown thatSAW devices can be used with zero-bias (normally ON) field effecttransistors (FETs) to produce wireless devices capable of readingvoltages generated by external sensors (such as AE sensors,thermocouples, etc.). The external sensor voltage is applied as the gateto source (G-S) voltage on the FET, and modifies the drain to source(D-S) resistance of the FET. This D-S impedance is electricallyconnected as a load impedance across a SAW transducer, and changes inD-S resistance alter the SAW response reflected from the loadedtransducer.

The hybrid FET/SAW sensor-tag wireless interface devices developedpreviously, and others using advantageous SAW embodiments according toaspects of the present invention described herein, can be used tomeasure electrical fields and magnetic fields, and by monitoring themagnitude and sign of the electrical field and the magnitude anddirection of the magnetic field, can provide information on the voltageand current in high voltage lines (and other current carryingconductors), and on the relative phase (leading or lagging) of thecurrent and voltage—providing information necessary to determine powerfactor. Such information can be useful, for example, in monitoring thecondition of the power lines and other electrical equipment.

Another type of system according to specific aspects of the presentinvention is a passive wireless power line voltage, current, andtemperature monitoring sensor system. This system utilizes SAW sensorsor sensor tags with field probes to measure the electric field andmagnetic field around current carrying conductors, which provide proxiesfor the voltage and current in the conductor. The distribution portionof the power grid runs essentially blind today, i.e., almost noreal-time data is available to the grid operator on the condition of thedistribution lines and transformers. Pinging smart meters can providedata on power outages at the individual meter level, but automateddiagnostic tools that inform operators about the details, locations, andcauses of outages are not available. Equally concerning, there are noprognostic tools for the grid that can predict component failures priorto problems occurring. Event driven condition based monitoring (CBM) ofthe distribution grid would enable operators to identify incipientfailures, such as transformers nearing failure, and prioritizepreventive maintenance to prevent outages. Since the cost of performingpreventative maintenance is much lower than that of responding to anoutage, such systems can reduce operating costs while enhancing gridstability. However, there are no current methods for distributedmonitoring of the grid that are low enough cost to enable widespreadmonitoring, particularly at the local distribution level. Low-costmonitoring systems that can be distributed along power lines down to anindividual span level of granularity could achieve unprecedentedsensitivity in monitoring grid conditions.

FIGS. 17A and 17B show various schematic representations of a voltageand current monitoring system 400 according to aspects of the presentdisclosure. FIG. 17A shows a cut-away view of the interior of thesystem's casing 408, which serves as mechanical housing for the currentsensors 406 and the voltage sensors 404, as well as providing a mountingmeans to attach the system to a current carrying conductor 402,including hot stick mounting for power lines. FIG. 17B shows the systemwith the mechanical housing 400 closed around the conductor 402. FIG. 18shows the concept for system operation on a three phase power lineaccording to aspects of the present disclosure. Each or the three phaseconductors has a wireless, batteryless sensor module that corresponds tothe system 400 shown in FIGS. 17A and 17B mounted on the line. Awireless interrogator, also referred to as a reader or a radio, ismounted on the pole below the power lines. The reader sends RF signalsthat activate the sensor systems on the lines, then receives anddigitizes the reflected sensor system RF responses. After local signalprocessing, sensor data can be transferred to the grid operator via thesmart grid network, or via cellphone car wireless mesh networks, or anydesired wireless communication system or protocol. A single reader cancollect and interpret data from multiple sensors (up to thirty-two ormore in the field of view of one interrogator). Moving all of thedigital signal processing (DSP) and wireless communications hardware toa reader that can be mounted in an accessible location away from thepower line or transformer being monitored reduces cost and enhancessystem reliability compared to systems with radios mounted on the highvoltage lines. The SAW devices used operate in harsh environments, andhave demonstrated lifetimes in excess of several decades in challengingenvironments.

How the SAW sensor-tag assembly modules measure voltage and current:Placing a loop of wire in (and perpendicular to) the magnetic fieldgenerated by a time varying current on an AC power line will cause acurrent to be induced in the wire loop. For an ideal open circuitedloop, this current induces a voltage across the open ends of the loop.By connecting the ends of the loop to a full bridge rectifier and thento the gate of a FET, a time varying drain to source FET impedance canbe generated as the current in the power line changes.

FIG. 19 shows a schematic representation of a wireless, batteryless SAWcurrent sensor-tag assembly according to aspects of the presentdisclosure. This assembly includes a magnetic field probe sensorconnected to a FET interface module, a SAW sensor-tag, and an antenna.The SAW sensor-tags provide a wireless interface with the FET(s),reading the D-S impedance that is being modified by the field probevoltage applied to the FET gate, thus monitoring the magnetic field andultimately the line current. In an alternate embodiment of the presentinvention, a Rogowski coil placed around the wire, with the gate andsource of the FET attached across the coil ends, can be used instead ofa simple loop shown in Figure A, making the voltage less dependent onpositioning of the coil relative to the power line. Using a FETinterface according to the present invention, the SAW sensor will beable to detect current magnitude and direction by detecting the strengthand orientation of the magnetic field produced. Data gathered by theinterrogator can include current magnitude and direction as a functionof time. This data can be collected for each phase of the power system,allowing extraction of the relative current amplitude and phaseinformation.

FIG. 20 shows a schematic representation of a wireless, batteryless SAWvoltage sensor-tag assembly according to aspects of the presentdisclosure. Once again, the assembly consists of a field probe that canproduce a voltage based on the electric field, a FET interface, a SAWsensor-tag, and an antenna, although in this case the field probe isdesigned to detect the electric field. Around an AC power line, atime-varying electric field will form with a field intensity that isdirectly related to the voltage on the line. Field intensity falls offapproximately as the inverse of the radial distance from the line. Byplacing two radially separate probes in the field, a voltage differencebetween the probes can be measured and used to drive a FET attached toone of the SAW sensor tags disclosed herein. As with the currentmonitoring case, the time varying impedance of the FET will load the SAWtag and allow wireless reading of the impedance of the FET. As a result,we can monitor the field intensity and by extension, the line voltage.Since the electric field falls off to ground over a distancecorresponding to the distance from the line to ground, which can varyfrom location to location and with different line types, the electricfield intensity at any given distance from the line will also vary withthese factors. Thus, measuring the electric field intensity will onlyprovide a scaled measure of the voltage on the line, unless acalibration can be done to establish the absolute voltage on the linerelative to ground.

A complete line monitoring sensor system (400 in FIGS. 17A and 17B) foruse on AC power systems requires that both current and voltage aremeasured during both positive and negative portions of the nominally 60Hz cycle, in order to allow determination of the direction of currentflow. Finding the relative phase between the current and voltage alsorequires accurate zero crossing detection for both signals. FIG. 21shows a SAW wireless interface device configuration with multiple FETsfor use in monitoring current (magnitude and direction) and voltage(magnitude and polarity) according to aspects of the present disclosure.The field probes that produce voltages V1 and V2 are loops (or aRogowski coil) for detection of time varying magnetic fields and simpleelectrical probes (similar to simple monopole antennas) for electricalfield measurement.

For field effect transistors (FETs), the Gate to Source (G-S) voltagecontrols Drain to Source (D-S) impedance of FET. Some FETs exhibit D-Simpedance characteristics that are desirable for use with SAW deviceswith positive G-S voltages, while others exhibit D-S impedancecharacteristics that are desirable for use with SAW devices withnegative G-S voltages, and yet others function with G-S voltages thatspan zero voltage.

For the SAW/FET configuration shown in FIG. 21, two voltages are pulledfrom the field probes (which measure the electric or magnetic field),producing a time-varying differential voltage of magnitude|V₁(t)−V₂(t)|, and diodes are used to control application of the probevoltages to one of two FETs depending on the sign of the differentialinput. On the positive half cycle of the line current or voltage (thewaveform shown), V1 is more positive than V2, activating the top diodeand applying V1 to the gate of the bottom FET, with V2 applied to theFET source. With proper FET selection, this applied voltage causes animpedance change in the drain to source of the lower FET, which modifiesthe reflectivity of the attached SAW transducer. Changes in the acousticwave signal reflected from the transducer can be interpreted as a changein FET resistance, or gate voltage, or ultimately electric or magneticfield strength. On the negative half cycle, V1 is more negative that V2,causing the top diode to shut off and the bottom diode to activate. Now,V2 is routed to the gate of the top FET, and V1 is applied to thesource. This modifies the drain to source impedance of the FET attachedto the top SAW transducer. In this way, the positive and negative halvesof an input differential signal from the field probes are separatelyidentifiable and measurable.

The discussion above assumes ideal diodes that have essentially zerothreshold voltage. The use of realistic diodes for input signal routing,however, impacts the performance of this approach, in that most realdiodes have non-zero threshold voltages (V_(TH)) that can be as much as0.7V (typically). This is the voltage at which the diode turns on. Thus,the voltages routed to the FET gates are lowered by the thresholdvoltage of the diodes. The zero voltage crossing point of the inputsignal also can become difficult to track due to the diode thresholdvoltage. This turn-on voltage could cause the circuit shown in FIG. 21to exhibit a ‘dead zone’ where the system is non-responsive fordifferential probe outputs from −V_(TH) to +V_(TH) (typically from about−0.7V to +0.7V), though careful selection of low threshold voltagediodes may reduce the dead-zone.

Since the SAW sensor-tag assemblies will be operating in a high voltagepower line environment, where transients are not uncommon, it isimportant to include protective circuitry. FIG. 22 shows the circuit ofFIG. 21, with added TVS devices across the two leads of the field probe.This ensures that, even if there is a surge in current or voltage on theline, the differential output of the field probe will not exceed apredetermined level.

A complete monitoring system 400 in FIG. 17 will include two SAWsensor-tag assemblies (shown in FIG. 22), one to monitor the linevoltage and one to monitor line current. In addition, one embodiment ofthe present invention includes a SAW temperature sensor to determine thepower line temperature. All of these sensors and sensor-tags can beimplemented on a single substrate, or alternatively it is possible toutilize multiple substrates used together to implement the sensor-tagassemblies in one embodiment of the present invention. Two or more diecan be used, potentially a reference die and one or more sensing die.The die can be mounted together in a common sample plenum, or thereference device can be hermetically sealed in one package while thesensing die is exposed to the media of interest in another package. Thereference die can be combined with one or more sensing die fortemperature or other parameters, and hermetically sealed in a package.This package can be electrically connected to an external sensing die,which loads the acoustic response on one of the sensing die or tracks.

The illustrations included herein are exemplary in nature, and do notencompass all aspects of the present invention. One skilled in the artwould recognize that the improvements provided by embodiments of thisinvention can be implemented to work with any of a wide range of knownSAW sensor and sensor-tag structures, including but not limited to thoseincorporating various diversity techniques (code, chirp, time, andfrequency diversity among others). A wide range of known codingtechniques can be implemented in combination with the embodimentsdescribed. It would be understood by one versed in the art that simpleon-off keying, phase modulation, pulse position modulation, and manyother techniques could be used with the techniques described herein toenhance the number of codes that work together without interference.Frequency diversity, code diversity, time diversity, and other knowntechniques can be combined to achieve sets of devices with desirableproperties. Any of these techniques could be utilized in theaforementioned device embodiments to increase the number of sensors thatcan work together in a system. Devices utilizing such structures couldalso be useful for RFID tag and sensor-tag applications, whereidentification of individual devices is desired. In addition,combinations of these techniques may be advantageous in certaincircumstances.

Other implementations of the invention will be apparent to those skilledin the art from consideration of the specification and practice of theinvention disclosed herein. Various aspects and/or components of thedescribed embodiments may be used singly or in any combination. It isintended that the specification and examples be considered as exemplaryonly, with a true scope and spirit of the invention being indicated bythe following claims.

What is claimed is:
 1. A surface acoustic wave sensor tag device,comprising: a piezoelectric substrate; at least one transducer arrangedon at least a portion of said piezoelectric substrate; and at least onesurface acoustic wave element formed on said piezoelectric substrate andspaced from said at least one transducer along a direction of acousticwave propagation, wherein said at least one surface acoustic waveelement includes at least one multistrip coupler; wherein at least oneof said at least one transducer or said at least one surface acousticwave element further comprises dispersive electrode structures; andwherein said at least one transducer further comprises a dispersivetransducer with a time symmetric dispersion pattern.
 2. A surfaceacoustic wave sensor tag device according to claim 1, wherein saiddispersive transducer with said time symmetric dispersion pattern islocated between two or more reflectors.
 3. A surface acoustic wavesensor tag device according to claim 2, wherein said two or morereflectors are comprised of ring shaped multistrip coupler reflectors.